Tin and tin alloys are most common solders used to provide solder joints in a wide variety of electronic devices, such as chips, crystal oscillators, lead frames, printed circuit boards, head gimbal assemblies (HGA) and the like. In addition to providing electrical connections, the solder joint provides a vital mechanical link between electronic devices. For example, in the HGA, solder ball is used to electrically and mechanically connect a head slider to a suspension of the HGA.
A myriad of solder structures have been proposed for the interconnection of one electronic component to another, and many interconnect technologies have known to us for forming those solder connection structures, such as surface mount technology (SMT), ball grid array or BGA technology, solder ball bonding (SBB) or gold ball bonding (GBB) technology and so on. Regardless of the form of the solder connection or the method of making the solder connection, there are typically three stages at which cleaning of the solder surface may be essential. First, during deployment of the solder prior to making the connections, processing of solder may leave undesirable residues which will interfere with proper solder wetting of solder pads and need to be removed. Second, to allow good wetting of the solder on the solder pads, flux is most often used and will need to be removed to avoid leaving corrosive contaminants on the electronic device. Finally, rework of part-good assemblies requires special handling which may require a cleaning step that assures the reliability of the package.
Unfortunately, however, removal of the flux and flux products and cleaning of the solder is a difficult task because the cleaning process may itself be corrosive to the solder joint and/or electronic device. Additionally, dissolution of the solder can occur resulting in a smaller amount of solder forming the solder joint and cause disposal problems since the solders are generally lead/tin alloys and their solutions pose an environmental threat if discharged.
The corrosion and dissolution of the solder joint during cleaning is circumstantiated hereinafter. As an example, the solder is tin alloy, and the cleaning agent is water-based solution. As illustrated by FIG. 1, the tin (Sn) is undergoing electro-chemical corrosion. Such tin corrosion process includes two simultaneous reactions. One reaction is tin dissolution in water, which is an oxidation reaction. In this reaction, the tin (Sn) serves as an anode and releases electrons to form stannous ion (Sn2+), which is expressed by equation (4). At the same time, another reaction, which is a reduction reaction, is proceeding. In this reaction, the oxygen gas that is dissolved in the aqueous solution serves as a cathode and reduces to hydroxyl ion (OH−) by neutralizing the electrons, which is expressed by equation (40). The stannous ion (Sn2+) and the hydroxyl ion (OH−) formed by the above two reactions react with each other and thus yield tin hydroxide (Sn(OH)2), which is expressed by equation (41). During drying process thereafter, as shown in FIGS. 2a-2d, the tin hydroxide crystallizes to become crystals on or holes in the solder joint surface (see equation (42)). These crystals or holes make the solder joint get grainier as it cools, thus a little movement can make cracks along the crystals and form a high resistance joint. Moreover, such structure of the solder joint may affect both the reliability and the testability of the finished electronic product. In addition, after tin dissolution and crystallization during aqueous cleaning or water dipping, the solder joint surface causes the finished electronic product poor in appearance.

Hence, a strong need has arisen for providing an improved method of treating the solder joint surface to overcome the above disadvantages.